The present invention relates to electronic color image capturing systems, and more particularly, to a color image capturing system that reduces undesirable artifacts, such as color fringing at the edges.
There are many applications where it is necessary to capture a color image of an object or scene of interest for display and/or storage. These include live video display systems, video cam corders, digital still cameras (DSCs), etc. The traditional techniques for color image capture in consumer electronic devices have limitations in the quality of the reproduced color images which can be understood by describing the operation of conventional DSCs.
In conventional DSCs a large number of charge-coupled devices (CCDs) are arranged in a matrix, which has either a square or a rectangular format. Most DSCs use a matrix or area-array of CCDs because of their ability to capture an image more or less instantaneously, much like a conventional film camera. The matrix of CCDs captures the entire image in one exposure, but it can require several exposures to build full color information. This is because the CCDs are monochromatic, i.e. they can only detect light intensity, not color. Therefore DSCs must use a color filtration system in conjunction with the CCD matrix in order to separate the incoming light into its red, green and blue (RGB); cyan, magenta, yellow (CMY); or other color components. There are several different ways that this color separation can be performed in a DSC.
A first conventional method of color separation in a DSC is the so-called xe2x80x9cone-shot, three-chipxe2x80x9d approach. All of the color information necessary to generate and store a still digital image is captured in one exposure. As the light enters the camera, a beam splitter, such as a prism, separates the light into its color components and a trilinear CCD chip is used to capture full RGB color component information. Three images, each representing one of the colors, are then realigned with the aid of firmware to form the full color digital still image. Because the human eye is most sensitive to the green spectrum of light, some three-chip DSCs are made with two of the arrays registering green color information and the third array is made up of a mosaic of red and blue filter elements. Since gaps in the red and blue information exist, interpolation is used to create additional color information.
A second conventional method of color separation in a DSC is known as the xe2x80x9cthree-shot, one-chipxe2x80x9d approach. With this approach the DSC uses a filter wheel through which three individual exposures must be made to record the RGB color component information for the digital still image. Three separate images are then combined using the firmware stored in the camera. Problems can arise with this approach where the images are improperly realigned. Also, variations in light emissions during the three exposures can alter the color balance of the final image. Also, the mechanical structures needed to rotate the filter wheel require additional space for an actuating mechanism and are subject to failures.
A third conventional method of color separation in a DSC is known as the xe2x80x9cone-chipxe2x80x9d approach. With this approach a single integrated circuit contains all of the CCDs and filter elements are utilized, one filter element of a given color be associated with each individual CCD element The red, green and blue filter elements are arranged either in a striped pattern or a mosaic pattern. Some of the one-chip DSCs have image sensors in which there are more green than red and blue filter elements in order to accommodate the fact that the human eye is more sensitive to green in the visual spectrum, as well as the fact that more green improves image resolution.
In the one-chip approach each photosensitive CCD element functions to capture a single pixel of a given color in the final digital still image. At any given pixel location, only one color of light can be sensed, namely, the color of the filter element at that location. Additional color information at that pixel location is lost and must be inferred from neighboring pixels through a process such as interpolation. This process can lead to problems if incorrect color information is assigned to pixels. This is usually most apparent around high contrast edge areas, such as black text, where colored fringes xe2x80x9cbleedxe2x80x9d into the text. One CCD array mosaic pattern frequently used in one-chip DSCs is the so-called Bayer pattern illustrated in FIG. 1.
Some conventional DSCs use a lower quality lens system or a dispersing element with no optical power such that the image is blurred. This allows light from a given location in the image to be spread across more pixels, compromising picture quality (resolution). Although the resulting digital still image may, or may not, be as clear, certain undesirable artifacts, such as color fringing at the edges, are reduced, making the image more attractive to the user.
It is therefore the primary object of the present invention to provide a color image capturing system from which reproduced images have reduced undesirable artifacts such as bleeding, fringing and blurring.
It is another object of the present invention to provide a method of capturing color images that reduces undesirable artifacts such as bleeding, fringing and blurring in the reproduced images.
In accordance with the present invention a color image capturing system includes an image sensor that generates output signals representative of an image of an object or a scene of interest. The image sensor includes a plurality of individual photosensitive elements arranged in an array. Each photosensitive element represents a corresponding pixel location in the image. A plurality of individual color filter elements are arranged in a predetermined pattern. Each color filter element is positioned adjacent a corresponding one of the photosensitive elements of the image sensor. An optical element shifts the light transmitted to the image sensor so that a portion of the incident light that would otherwise fall on a single photosensitive element is divided between at least two adjacent photosensitive elements. Blurring and undesirable artifacts such as color fringing at the edges are thereby reduced in the reproduced color images.
The method of the present invention involves transmitting light reflected by an object or scene of interest onto an image sensor. The image sensor includes a plurality of individual photosensitive elements arranged in an array, each photosensitive element representing a corresponding pixel location in the image. Before the light strikes the image sensor it travels through a plurality of individual color filter elements arranged in a predetermined pattern. Each color filter element overlies a corresponding one of the photosensitive elements of the image sensor. The light is shifted by a coated mirror, prism or other suitable optical element before it reaches the color filter elements so that a portion of the light that would otherwise fall on a single photosensitive element is divided between at least two adjacent photosensitive elements. The output signals from the image sensor are processed and a color image is reproduced. Blurring and undesirable artifacts such as color fringing at the edges of the reproduced color image are reduced.